How can I control the size (Diameter) of a ring that is made with the simple transform / Bend?
Different, slightly quicker approach -

Add a cylinder and change the radius and depth in the last operator panel to match the desired values:


Then in edit mode, select the top and bottom faces, extrude in place (press E, then immediately after Enter before the faces had a change to move anywhere), then change the pivot point to individual origins:


Now scale the top and bottom faces to match the desired radius. Since we change the pivot to individual origins, each face will scale around its own origin, and in effect both faces will be inset.


Now the only thing left is to bridge the two faces to create the inner hole and geometry (see answer above to explain how to activate the looptools addon if youre not familiar with it):